Various processes for preparing polyvinyl butyral are well known in the art. For example, French Patent 2,401,941, which is incorporated herein by reference, discloses a process in which polyvinyl alcohol is dispersed in an aqueous solution and reacted with butyraldehyde in the presence of an acid. In particular, an aqueous solution containing polyvinyl alcohol of 8 to 15 percent by weight is reacted at a temperature of from 5.degree. to 12.degree. C. with butyraldehyde in the presence of an acid and an emulsifier. A sufficient amount of butyraldehyde is used in order to react 75 percent to 88 percent of the polyvinyl alcohol in the reaction mixture. This amount is added to the mixture gradually so that the polyvinyl butyral precipitates in the form of particles 10 to 90 minutes after the start of the reaction. The reaction mixture is then agitated for more than 30 minutes at a temperature of 8.degree. to 15.degree. C., after which the reaction process is accelerated by an increase in temperature to between 60 .degree. and 80.degree. C., for 11/2 to 4 hours. Subsequently, a base is introduced into the reaction mixture to neutralize it to a pH level between 9 and 11 (hereinafter referred to as the "neutralization process"). The reaction is then continued for more than 1/4 hour at this temperature. The after-treatment of the polyvinyl butyral is completed by the separation of the final product by filtration, followed by a thorough washing with water.
When the high-quality polyvinyl butyral obtained from the above processes is admixed with a plasticizer, a polyvinyl sheeting results having properties satisfactory for the subsequent handling of the sheeting and performance in a laminar structure such as laminated safety glass. These properties include a reduced tendency to compression flow; exceptional laminar and support strength and impact resistance (e.g., in falling ball tests); enhanced bonding properties to contiguous glass surfaces (e.g., in pummel tests); high transparency properties, and high rigidity.
However, the drawbacks of these processes can be seen when the emulsifier used is a sodium alkylsulfonate or sodium alkylaryl-sulfonate, such as sodium dodecylbenzene-sulfonate or sodium arylsulfonate. These emulsifiers are necessary to prevent an agglomeration of polyvinyl butyral particles which would greatly diminish the optical quality of the polymer product. Unfortunately, if these emulsifiers are left in the polyvinyl butyral product, they also reduce the product's ability to adhere to glass. As a result, these emulsifiers must be removed in the after-treatment of the polyvinyl butyral product. Typically, this after-treatment is carried out in an aqueous medium under basic conditions, namely, at a pH between 9 and 11. Most frequently, soda is used in this after-treatment to neutralize the acid of the catalyst to the desired pH level. Unfortunately, soda or other substitute chemicals required for the neutralization process significantly increase the cost of the after-treatment process of polyvinyl butyral. In addition, this process prolongs the already lengthy after-treatment process of polyvinyl butyral. Moreover, during after-treatment, the butyraldehyde is condensed to form a number of precipitates, including ethyl-2-hexenal, which has a very unpleasant odor.
Additionally, sheets obtained by the above processes are not sufficiently rigid. This makes them difficult to handle by automatic handling and cutting machines which position and tailor these sheets as required during the manufacture of glass sheets.